Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a small volume and does not need to be stored at high pressure.
Usually, natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stream suitable for liquefaction at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas is then further cooled and expanded to final atmospheric pressure suitable for storage and transportation.
In addition to methane, natural gas usually includes some heavier hydrocarbons and impurities, including but not limited to carbon dioxide, sulphur, hydrogen sulphide and other sulphur compounds, nitrogen, helium, water, other non-hydrocarbon acid gases, ethane, propane, butanes, C5+ hydrocarbons and aromatic hydrocarbons. These and any other common or known heavier hydrocarbons and impurities either prevent or hinder the usual known methods of liquefying the methane, especially the most efficient methods of liquefying methane. Most known or proposed methods of liquefying hydrocarbons, especially liquefying natural gas, are based on reducing as far as possible the levels of at least most of the heavier hydrocarbons and impurities prior to the liquefying process.
Hydrocarbons heavier than methane and usually ethane are typically condensed and recovered as natural gas liquids (NGLs) from a natural gas stream. The methane is usually separated from the NGLs in a high pressure scrub column, and the NGLs are then subsequently fractionated in a number of dedicated distillation columns to yield valuable hydrocarbon products, either as product steams per se or for use in liquefaction, for example as a component of a refrigerant.
Meanwhile, the methane from the scrub column is subsequently liquefied to provide LNG. Pressure reduction and separation such as ‘end flash’ after liquefaction can provide a gaseous methane recycle stream.
U.S. Pat. No. 4,541,852 describes a system for liquefying and subcooling natural gas in which compression power is redistributed from the closed cycle refrigerant by subcooling the LNG and reducing the pressure and flashing the LNG to recover a gaseous phase natural gas. The gaseous phase natural gas is then recompressed and recycled to the feed of the system.
The system of U.S. Pat. No. 4,541,852 requires the recompression of the gaseous phase natural gas from the depressurisation and flashing of the LNG to the feed stream pressure of 815 psia. A high power recompressor driver is therefore required.
The system of U.S. Pat. No. 4,541,852 does not include an NGL extraction system. Thus, it is not possible to alter the specification of the LNG product by removing NGLs from the feed stream. Any hydrocarbon components in the feed stream which may solidify during liquefaction may cause plugging in the system.